I disagree that the comma is optional. I agree that its use has become generally accepted in informal writing, but I don't think it can be called "correct."
Most importantly, it differs entirely from the thing it's mimicking:
Dear, in this sense is more of a title and less a greeting.
In the message, hi is addressed to Xxxl, and it therefore should get a comma to indicate that part 1 addresses part 2.
Knowing entirely that the following is unconvincing, I'll offer one more point: Think of our intonation as you would read it aloud.
The years have seemed like hours since we [...]
Hi, Jim --
How goes it on the chicken farm?
In the first, the tone is even or falling throughout; in the second, it does that addressing comma thing (roughly, down on first word, up on second) along with the little stutter pause that others have referred to.